1. Field of the Invention
The present invention relates to a multiphase DC motor and starting method thereof, and more particularly to an improvement for improving its starting characteristics.
2. Related Art
A multiphase DC motor has been conventionally used as a motor for rotationally driving a magnetic disk unit. This type of motor is also referred to as a spindle motor, and it is well known that a spindle motor comprises a stator having stator coils supplied with exciting currents, a rotor having a rotor magnet generating a rotational force from electromagnetic interaction between the rotor magnet and the stator coils, and a position detecting sensor for detecting the rotational position (rotational angle) of the rotor magnet.
According to this type of motor, the rotational angle of the rotor magnet is detected by a position detecting sensor, and exciting currents to be supplied to the stator coils are switched in response to the detection signal from the position detecting sensor. A Hall element is used as the position detecting sensor.
In recent years, a so-called sensorless multiphase DC motor has become popular in order to minituarize the motor and to prevent characteristics of the sensor from being deteriorated. This motor detects the position of the rotor magnet based on induced voltages in a coil in which exciting currents are not flowing, instead of using the position detecting sensor.
No counter electromotive force can be obtained when the motor is stopped. Accordingly, in the sensorless motor, the rotor is swung when the motor is started. For example, in a three-phase spindle motor, a stepping process for sequentially supplying the exciting currents is repeated. During the stepping process, forward, resting and reverse exciting currents are supplied to respective phases.
However, such a multiphase DC motor has the following technical problem, particularly with respect to its starting method.
According to the multiphase DC motor, the position of the rotor magnet is detected due to the induced voltage. However, neither the voltage is induced nor the polarity of the magnet is known when the motor is stopped. Accordingly, the motor is forcibly started by generating a signal having a predetermined pattern. However, the motor may not start due to a low torque depending on the position of the rotor, or the rotor may be rotated reversely due to a magnetic field generated in a reverse direction when powered.
In order to solve the above problem and improve the starting reliability, the inventor of the present invention has proposed a novel starting method for a sensorless motor as disclosed in U.S. Pat. No. 5,235,264. This starting method includes the step of reversing, in the starting time, an exciting current from the positive direction to the negative direction or vice versa without including the rest time. When the reversing step is performed, the magnetic flux changes drastically to produce a high torque. As a result, the starting reliability is improved.
However, in the above starting method, the reversing step is performed only at one phase of the coil at a time so that a sufficient torque cannot be obtained. Thus, more improvement has been required. In addition, the time required for starting the motor is relatively long, and thus the reduction of the starting time has been demanded.